Double Opt-in for GDPR Compliance

Please add Double Opt-in functionality to SureContact. This is essential for GDPR compliance in the EU, especially Germany, where it's legally required for email marketing. Without it, businesses risk significant fines and legal issues. Double Opt-in ensures subscribers confirm their email address before being added to lists, which is the standard for EU data protection laws. This would make SureContact a complete solution for European markets.